Method of heat treating beryllium copper alloys



E. M. SMITH `lune 23, 1964 METHOD OF HEAT TREATING BERYLLIUM COPPERALLOYS Filed March 19, 1962 INVENTOR. 2mn/@ew M 9M/7H ATTRNEK mm w Q wNWN NN United States Patent O 3,138,493 METHOD F HEAT TREATING BERYLLUMCPPER ALLOYS Ellsworth M. Smith, Reading, Pa., assiguor to The BrushBeryllium Company, Cleveland, Ohio, a corporation of Ohio Filed Mar. 19,1962, Ser. No. 180,652 11 Claims. (Cl. 14S-11.5)

This invention relates generally to a method of heat treating berylliumcopper alloy sheet material and particularly to a method o f heattreating beryllium copper alloy sheet material in a continuous operationunder controlled time and temperature conditions.

The invention is herein described and illustrated in connection with thetreatment of beryllium copper alloy sheet. The term beryllium copperalloy is used herein to denote alloy consisting essentially, by weight,of from 1.4% to 2.1% beryllium, 0.17% to 0.42% of metal selected fromthe group consisting of cobalt, nickel, and mixtures thereof, and thebalance copper.

The term sheet is not used herein in a specic sense as defined intechnical handbooks and standard specifications, but in a more generalsense to include not only metal stock classied as sheet, but also thatclassified as thin plate, strip, and foil.

It also applies whether or not the stock is in the form of a band, suchas is commonly wound into coils as it leaves the rolling mill, or in theform of cut-off lengths of such a band, and whether or not it is incondition for fabrication, or already fabricated into articles.

One of the earliest prior methods of producing such sheets is to formthe beryllium copper alloy into sheets, cold work the sheets, and thensubject them to a hardening operation by passing them through a mulefurnace wherein they are heated to a temperature of about 600 to 650 F.and maintained at that temperature for a relatively long period of abouttwo to three hours. This method is used on the sheets whether they arein flat form as received from the mill or are in the form of articlesfabricated therefrom. Due to the great length of the heating periodrequired in this prior process, high internal stresses are developed inthe sheets and limit their formability. Additionally, the desiredphysical and mechanical properties cannot be obtained consistently. Theresultant product tends toward brittleness. However, when the materialis treated in this manner in sheet form, the sheets must be retained infixtures during treatment to prevent distortion.

A second method is one wherein the beryllium copper alloy sheets arecold worked in the form of strip to a maximum reduction of 37% afterwhich the strip is heated in coiled condition at a temperature range ofabout 600 to 850 F. for a relatively long period of from one hour toseveral hours, to provide the desired tensile strength. The stripresulting from this method evidences an advanced state of gamma growth,particularly at the grain boundaries, and this causes variations inhardness throughout the coil or strip, reduces the endurance life of thestrip stock and resultant sheets, and decreases their formability. Inaddition, the still lengthy heating cycle of the sheets in coil formresults in what is commonly referred to as coil set; that is, the sheetstend to retain the curvature which they had While forming a part of thecoil. The sheets so curved are detrimental to high speed pressperformance and result in non-uniformity of the fabricated part.

A third method also is employed. This method comprises cold working ofthe material in sheet form to between about 37% reduction and 60%reduction to develop a sheet which can be aged at high temperatures in arelatively shorter time than employed in the other two methods. Thematerial is then passed continuously in strip form through a mulefurnace, in which a reducing atmosphere is maintained, for a period ofabout three minutes to fifteen minutes, and at a temperature of fromabout 725 to 800 F. The method has a number of advantages in that thetime required for heat treatment is t greatly reduced and the sheets arefree from coil set and have excellent endurance life. However, becauseof the extensive reduction by cold working, the resultant sheets havehigh residual internal rolling stresses and unidirectional graindistribution which limit their formability. Likewise, the resultsproduced by the method are not consistent in that the physicalproperties of the resultant sheets vary frequently and uncontrollably.

Currently, in the commercial preparation of beryllium copper alloysheets, the beryllium copper alloy materials are cast into ingots. Theingots are first hot rolled and then alternately solution annealed andcold rolled until a desired and predetermined ready to nish thickness isobtained. The strip is finally cold rolled to its nal thickness, and mayor may not be annealed thereafter, depending on customer requirements.

Insofar as concerns beryllium copper sheets products commonly sold tothe customer for fabrication, no standard of physical propertiesrequired has been established by the government or any recognizedtechnical authority other than for beryllium copper alloys containingfrom 1.80 to 2.05 percent, by weight, of beryllium. This standardestablished for this alloy is applicable to sheets supplied by the millin solution annealed and cold rolled tempers, and also to sheets whichhave been heat treated in either strip or fabricated form at 600 F. forabout two to three hours. This method of treating the sheets in the formof fabricated parts is performed by the customer, and while it developshigh tensile strength, it also renders the parts brittle. In the eventthe sheets are given a heat treatment at 600 F. for two to three hours,the

'resulting brittleness would preclude fabrication because the sheetwould crack and fracture during forming operations. Thus, the customeris confronted with the problems of heat treating the sheets offabricated form and of providing and employing various expensive jiggingfixtures to minimize distortion during the heat treatment.

It became apparent, therefore, that by improving suiciently theformability of mill heat treated sheets they could be fabricated intoparts without fear of developing forming cracks, and without requiringheat treatment by the fabricator before or after fabrication.

A principal object of the present invention is to provide a method oftreating beryllium copper alloy sheets of this general character toimpart thereto physical properties which sufliciently improve theirformability to make possible their fabrication by the fabricator withoutthe necessity of any heat treatment at the fabrication site, eitherprior or subsequent to the fabrication of the sheets into article form.

Another object is to provide a method of heat treating 3 such sheets soas to improve their elongation, and also, under certain conditions, toimprove their tensile strength, or both their tensile strength and theirelongation concurrently.

A more specific object is to heat treat such sheets, in either strip orfabricated condition, to effect stress relief and thereby improve therelationship of their elongation to their tensile strength.

Another important object is to improve the physical properties ofberyllium copper alloy sheets by heat treatment of the sheets for anextremely short interval, as compared to the relatively long periodsheretofore employed, and by quick quenching of the heated sheets.

A further object is to provide such a method which is adapted for bothcontinuous or batch type operations.

Other objects and advantages will become apparent from the followingdescription wherein reference is made to the drawings, in which:

FIG. l is a top plan view of a treating apparatus for practicing themethod of the present invention on two strips of metal concurrently, thecover being omitted for clearness in illustration;

FIG. 2 is a side elevation of the apparatus illustrated in FIG. l, partsthereof being shown in section;

FIG. 3 is a fragmentary vertical longitudinal sectional view of the bathcontainer of the apparatus, and is taken on the line 3-3 of FIG. l; and

FIG. 4 is an enlarged vertical cross sectional view taken on the line4-4 in FIG. 1.

FIG. 5 is a vertical cross sectional view taken on the line 5--5 in FIG.l.

Referring to the drawings, two sheets 1 of beryllium copper alloy,Wrapped into the form of coils 2, are supported on suitable rotatablecoaxial pay-off supporting spindles 3 for permitting the sheets to bedrawn continuously therefrom through the treating apparatus from rightto left, FIG. 1. Each strip 1 is fed from its coil 2 through suitabletension rolls 4, over a suitable guide roll 5, and under a supplementalpositioning roll 6, and thence into a bath of molten salt contained inan elongated container 7. The strip is held immersed in the bath bysuitable holddown rolls S so as to be maintained at all times below thelevel of the bath, later to be described.

The strip passes from the left hand holddown roll 8 into a suitablequenching compartment 10 wherein it is cooled by spraying liquid coolantthereon by means of spray nozzles 11. From the cooling compartment 10,the strip passes between pinch rolls 12 over a guide roll 13 and isrewound into a coil 14.

The pinch rolls 12 are driven through a clutch 15 by a sprocket 16,chain 17 and driving sprocket 18. The sprocket 18 is driven by a motor19 of a recoiler 20. The pinch rolls 12 pull the strip from the coil 2through the bath, and maintain the strip under tension and in ahorizontal plane passing between, and tangent to, the rolls 12, while itis immersed in the bath. Each coil 14 is rewound on its recoiler 20after passing the pinch rolls.

As mentioned, the bath container 7 is elongated. It is in the form of anopen top tank 21 having a removable cover 21a with openings 22 at theends. Within the tank 21 are two troughs 23 which are laterally spacedfrom each other, extend lengthwise of the tank 21, and terminate attheir ends a short distance from the ends of the tank 21. The troughs 23have side walls 23a and end walls 23b. Each end wall of each trough hasan upper edge 23e and terminates at its upper edge below the upper edgesof the side Walls 23a. Each edge 23C provides a weir over which themolten salt bath, being introduced continuously into the troughs,discharges into the end portions of the tank 21.

Beneath the tank 21 is a heating tank 25 wherein are heating coils 26through which a heating medium from any suitable source is circulatedfor heating the salt of the bath to the required temperature andliquidity.

Within the tank is a feed pipe 27 which has an inlet end 28 opening intothe tank 25 and an outlet end 29 for supplying molten salt to thetroughs 23. The molten salt is drawn into the inlet end 28 and forcedout of the outlet end 29 by means of a recirculating pump. The pump isin the form of a propeller 30 mounted on a vertical shaft 31 and drivenrotatably through the medium of a belt 32 by a motor 33. The pump actsto pump the molten salt of the bath continuously into the troughs 23. Abaffle housing 34 is disposed above the outlet end 29 so as to preventthe splashing of the molten salt. The housing has passages 35, as bestillustrated in FIG. 4, through which the salt ows from the outlet end 29into the troughs 23.

The molten salt discharged over the weirs 23e falls into the endportions of the tank 21, which provide catch basins 36, and thence owsfrom the basin 36 along the bottom of the tank 21, beneath both thetroughs 23 and the space therebetween, and discharges from the tank 21into the heating tank 25 for reheating and recirculation. The bottomwall of the tank 21 slopes downwardly from its ends to the tank 25 toassist in the return flow of the molten salt. The openings 22 aredisposed at levels such that the strip 1 can enter the tank 21 throughthe opening 22 and pass over, and in spaced relation to, the weirs 23Cof the troughs 23, below the level of the overflowing bath, and thencethrough the trough 23, in spaced relation above the weirs 23e at theoutlet ends of the troughs into the compartment 10, and therethroughonto the coil 14, preferably being maintained in tensioned condition andhorizontal from its point of initial immersion to the pinch rolls 12.

In operation, therefore, the strip is fed from the coil 2 throughsuitable tension rolls 4 and over a guide roll 5, and under asupplemental roll 6 into the salt bath in the tank 21, wherein it ismaintained immersed by virtue of the holddown rolls 8 and the pinchrolls 12. It passes through the bath and into the cooling compartment 10wherein it is sprayed with coolant. It passes from the compartment 10through power driven tension rolls 12, over idler guide rolls 13, ontothe coil 14 on the recoiler 2t). By the action of the tension rolls 4and 12, the sheet is held straight, fiat, horizontal, and under tensionas it passes through the bath, as hereinbefore described. AS a result,the surface of the coiled sheet is smooth and of straight cross sectionat all points along its length.

If extremely heavy sheet stock is to be processed, it is not necessaryto pass it over the guide rolls 5 and 13. Instead it can pass from thecoil 2 through the tension rolls 4 and over the supplemental positioningroll 6, which is lowered for this purpose to guide the strip properlythrough the inlet opening 22.

The speeds, time and temperatures to be employed will be varied to someextent depending upon the sheet or strip thickness, the particular rangeof metal ingredients of the alloy being treated, and the physicalproperties desired in the finished sheet.

In accordance with the present invention, beryllium copper alloy sheetsare heated by immersing them in the molten salt bath while the bath ismaintained at a temperature ranging from about 525 F. to 825 F. Theimmersion is for an interval from as little as two seconds up to aboutsix minutes. This can be determined by the speeds of the rolls 12 and bythe position of the rolls 8. The closer the spacing of the rolls 3 fromeach other, the less is the time of exposure of the sheet to the bath.

The heated sheets are quenched in the compartment 10 by sprayed coolantwhich is maintained at a temperature of about 40 F. to 70 F., this beingadequate to cool the sheets almost instantly.

Strip treated in this manner, while thus held under tension by therolls, is dead Hat; that is, it is substantially planar except insofaras it is subsequently wound into a coil 14, and even after coiling ittends to resume its at condition when uncoiled.

If desired, the sheet stock may be cut into separate, sheets andconveyed through the bath by conventional conveying means, but this isnot as satisfactory inasmuch as it must be performed by equipmentrequiring considerable servicing and further does not permit oftensioning the sheets while they are passing through the bath and whichis desirable.

The salt bath employed in the process comprises a mixture of neutralinorganic nitrate and nitrite salts which are employed in molten stateand in such state are unreactive with the sheets. Many commerciallyavailable neutral salt mixtures used for standard salt baths can beemployed, but the nitrate-nitrite mixture is preferred as it will notreact with, and has no deleterious effect upon, the beryllium copperalloy sheets. The particular type of such salt chosen is dependent uponthe heating range desired.

The quenching bath may comprise a fluid spray, such as air or water. Acoolant liquid bath with sheet immersion may be used if desired.

Within the broader ranges of times and temperatures hereinbefore setforth, certain different eifects can be obtained by variations of timesand temperatures within narrower ranges, but the most outstandingdifference is that resulting from the variations in the time element.For example, the results desired may be stress relief, with anoutstanding increase in percent elongation without any diminution orincrease in tensile strength. On the other hand, it is quite oftendesirable to increase greatly the tensile strength and, while this isfrequently accompanied by a diminution in elongation, nevertheless amuch higher elongation is retained in a higher range of tensile strengththan has heretofore been possible. Finally, in some instances, not onlycan the elongation be increased substantially, but the tensile strengthcan be greatly increased concurrently therewith.

The various physical properties desired are to some extent controlled bythe needs of the customer. However, the important feature is that by thepresent method the eifects obtained can be preselected within certainnarrow limits within the broader limits and consistently. producedwithin those narrow limits.

The following tables disclose a number of dilerent characteristics ofsheets for customer use supplied by the mill in accordance with thepresent method. The data in all tables below were obtained by testing0.20 inch thick beryllium copper alloy sheets produced in accordancewith the prior method above disclosed, and then heat treated inaccordance with the present invention. Similar data and results havebeen obtained by testing beryllium copper strip ranging from about 0.07inch to about 0.0005 inch inthiclmess.

Table I Contz'nuous Strand Mill Heat Treating in a Salt Bath Alloy AAlloy B Age Hardening Conditions Be 2.03%, Be 1.61%, Initial Temper Co0.25%, Co 0.23%,

Balance Copper Balance Copper Temper- Tensile Percent Tensile PercentTime ature, F. Strength, Elong. Strength, Elong. p.s.i. in 2in. p.s.i.in 2in.

73, 000 40 73, 000 35 700 74, 000 42 79, 000 35 700 75, 000 37 84, 00032 180 sec 700 90,000 24 93, 000 31 30 see 750 73, 000 40 76, 000 36Annealed sec 750 79, 000 32 86, 000 27 sec 750 110, 000 19 100,000 19300 sec 750 143, 000 12 136, 000 14 30 see 800 74, 000 39 77, 000 35 60sec 800 85,000 28 87,000 30 180 sec 800 115,000 12 108, 000 18 ASTMStandard-AT 3 hrs. 4 0 20 85, 000 13 26 90, 000 26 60 sec 700 100,000 2195,000 22 180 sec 700 157,000 9 112, 000 24 30 sec 750 100, 000 16 93,000 22 M H10% Reduction in Thickness 60 sec 750 121,000 14 100, 000 18180 sec 750 169,000 9 120,000 14 8 133, 000 11 14 95, 000 23 14 103, 00018 7 119, 000 14 ASTM Standard-M HT 3 21 97, 000 21 60 sec 16 104,000 19180 sec 700 152, 000 9 124,000 16 30 seo 750 108,000 18 103, 000 19Elf-20% Reduction in Thickness- 60 sec 750 126, 000 14 110,000 15 180sec 750 169,000 8 132, 000 13 300 sec 800 181,000 6 147,000 10 30 sec800 121,000 13 105,000 20 60 sec 800 135,000 14 113,000 14 180 sec 800152,000 10 129, 000 13 ASTM Standard% HT 2 0 3 104, 000 5 9 119, 000 146 133, 000 11 4 142, 500 9 6 125, 000 11 Hard-37% Reduction InThickness., 5 136,000 8 4 157, 000 6 3 163, 000 6 6 131, 000 11 6 151,000 6 7 154, 000 6 ASTM Standard-HT 1 Table II.--Continuous StrandStress Relieving zn a Salt Bath-Alloy A Stress Relieving FederalSpecifications Conditions QQ-C533 and ASTM Tensile Percent InitialTemper Strength, Elong,

Temperp.s.i. in 2 in. Tensile Percent Time, ature Strength, Elong. scc.I". p.s.i. in 2in.,

nun.

73, 000 40 600 73,000 43 600 73, 000 44 600 73,000 45 600 73,000 45Annealed 600 73,000 43 60, 000-78, 000 35 5 700 73,000 42 10 700 73, 50043 15 700 74, 000 44 30 700 74,500 42 45 700 74, 500 39 0 M 85,000 22 5600 85,500 26 10 600 85, 500 28 15 600 86, 000 30 30 600 87,000 30 A1Il0% Reduction in Thickness 45 600 88,000 29 75, 000-88,000 10 5 70086,000 24 10 700 86,500 26 15 700 88, 000 28 30 700 91,000 26 45 700 94,500 22 0 91, 000 16 5 550 91, 500 18 10 550 92, 000 19 15 550 93, 000 1830 550 93, 500 20 45 550 95,000 20 s as 92,233 17 93, 21 $4 1I 20%Reduction 1n Thickness. 15 600 93' 000 22 85,000-100, 000 5 30 60095,000 22 45 600 97, 500 21 5 700 93,000 22 10 700 94,500 23 15 700 94,500 22 30 700 97,000 21 45 700 101,000 18 0 110,500 3 5 550 111,000 7 10550 111,000 8 15 550 111, 090 9 30 550 111, 000 9 45 550 112, 500 9 13se 1121033 1 11 ,5 11 Hard 31% Reduction in Thickness 15 600 111,000 13100,000 120,000 l Table Ulf-Tensile Strengths Obtaz'nable byConventional Mill Processing of Alloy A Rolling Reduction, percent Rangeof Tensile Strength (psi.)

Percent of Elon- Temper Designation gatlon 10D-110, 000 110-120, 000120-135, 000 135-150, 000 1GO-170, 000

Table IV.-Tensile Strengths Obtanable Only by Salt-Bath Processing ofAlloy A Rolling Range of Percent Tempel' Designation Reduction, Tensile0i Elonpercent Strength gation (ps1.)

XHM "190'Y 10 1GO-170,000 8 XHMS "190 10-20 170190,000 5 Table I, forexample, relates generally to sheets in which tensile strength was to beemphasized, either with an improvement, or in some cases a slightdiminution, of the present elongation. The results are tabulated fordifferent mill hardened tempers obtained by predetermined reduction bycold rolling.

Table II is directed more to stress relief whereby the elongation isgreatly increased while maintaining the tensile strength within theFederal Government and A.S.T.M. specifications for the respectivetempers.

Table I covers two alloys, Alloy A containing 2.03% of beryllium and0.25% of cobalt, balance copper, and the Alloy B containing 1.61% ofberyllium and 0.23% of cobalt, balance copper, as noted at the heads ofthe columns in the tables. Table II relates only to the former one ofthe alloys of Table I.

It is apparent, therefore, that in accordance with the present method,greatly improved characteristics, particularly in the relation ofelongation to tensile strength, can be obtained in heat treating periodsconsiderably shorter than those heretofore employed with like startingsheets. In general, the physical properties described are obtained withsheets in which the reduction generally does not exceed 37%, and usuallyis considerably below that amount, as compared to prior methods whereinthe physical properties are improved by reductions above 37% to as muchas 60%.

Table III shows the reductions which are employed in conventionalprocesses and the corresponding tensile strength as to Alloy A only.

Table IV, on the other hand, shows like information, as to Alloy A,resulting from the present process. Therein, tensile strengths in arange of 160,000 to 170,000 pounds per square inch, starting with asheet having temper of only reduction, are obtained, as compared Ito theextra hard temper of Table III, requiring a 60% reduction in thestarting sheet.

By employing 10% to 20% reduction, further increases in the tensilestrength into a range of from 170,000 to 190,000 pounds per square inchare obtained by the present method. Thus, by the present method,employing the salt bath, reliance for high tensile strength does nothave to be placed on high degrees of reduction in the thickness of thestarting sheets, with the corresponding high residual internal rollingstresses resulting therefrom, in the nal sheets. Since residual rollingstresses are maintained at a low level, a corresponding high percentageelongation is obtained after the salt bath heat treating.

Since the process dispenses with the necessity for the heavy reductionsand lengthy heat treatments, it makes possible the economical productionof materials with much higher elongations at both the high and lowtensile strengths than heretofore have been available.

Having thus described my invention, I claim:

1. The method of producing beryllium copper alloy sheets having apredetermined tensile strength and elongation and comprising coldrolling of a sheet consisting, by weight, essentially of from 1.4% to2.1% beryllium, 0.17% to 0.42% of metal selected from the groupconsisting essentially of cobalt, nickel, and mixtures thereof, and thebalance of copper, to effect a predetermined reduction and thereby toincrease the tensile strength, whereby a concurrent reduction in percentelongation occurs, then heating the rolled sheet in a salt bath to atemperature ranging from about 525 F. to 825 F., and maintaining thesheet heated in said temperature range in said bath for a period fromabout two seconds to about five minutes, then removing the sheet fromthe bath, and immediately upon removal, quenching the sheet with aliquid bath, while maintaining the sheet under tension continuouslythroughout the heating and quenching operations.

2. The method according to claim 1 wherein said reduction of said sheetis a maximum of about 37%.

3. The method according to claim 1 wherein said salt bath isnon-reactive with the sheets.

4. The method according to claim 3 wherein said salt consistsessentially of a mixture of inorganic nitrites and nitrates.

5. The method according to claim l wherein the sheets are conveyedcontinuously along a predetermined path and, during the continuoustravel along said path, are subjected successively to said heating inthe salt bath, removal from the bath, and quenching.

6. The method according to claim 1 wherein said period is from twoseconds about forty-tive seconds.

7. The method according to claim 1 wherein strength is further increasedby maintaining the material in said bath for a period of time greaterthan that required to produce the maximum increase in percentelongation.

8. The method according to claim 7 wherein the longer time of heating isdiscontinued while the elongation remains greater than it was at thebeginning of the heating operation.

9. The method according to claim 1 wherein the alloy consists, byweight, essentially of beryllium about 1.80% to 2.05%, metal from thegroup consisting of cobalt, nickel, and mixtures thereof, from 0.20% toabout 0.27%, and the balance being copper.

10. The method according to claim 1 wherein the alloy consists, byWeight, essentially of beryllium about 1.6% to 1.8%, metal selected fromthe group consisting of cobalt, nickel, and mixtures thereof, of fromabout 0.20% to 0.35%, and the balance being copper.

1'1. The method according to claim l wherein the temperature to whichthe sheet is heated is of from about 700 F. to 800 F.

References Cited in the lile of this patent UNITED STATES PATENTS1,321,530 MacDonald Nov. 11, 1919 1,570,815 Wylie Jan. 26, 19261,635,793 Koref et al. July 12, 1927 1,712,663 Gero May 14, '19291,852,528 Kinney Apr. 5, 1932 2,172,639 Werner Sept. 12, 1939 2,192,495Werner Mar. 5, 1940 2,279,684 Johnson Apr. 14, 1942 2,717,845 CarterSept. 13, 1955 2,818,075 Dunlevy et al. Dec. 31, 1957 UNITED STATESPATENT OFFICE CERTIFICATE OF CORRECTION Pabenb NQ., 3,138,493 June 23,1964 Ellsworth lVL, Smith ib -ie hereby Certified that errer appears intbe above numbered paben't requiring correction and that the saidLetters Patent should reedv as corrected below.

Columns 5 and q in Jbhe table, under the heading "l irl-20% Reduction inThicknessu and opposite "300 sec" in the first ColumnY the figure in thenext Columnv under `ehe heading 'l"em1oerature OFI." for "800" read 750Signed and sealed this 3rd dey of November 1964:7

(SEAL) Amst:

ERNEST W, SWIDERA EDWARD J. BRENNER Attesting Officer Commissioner ofPatents

1. THE METHOD OF PRODUCING BERYLLIUM COPPER ALLOY SHEETS HAVING APREDETERMINED TENSILE STRENGTH AND ELONGATION AND COMPRISING COLDROLLING OF A SHEET CONSISTING, BY WEIGHT, ESSENTIALLY OF FROM 1.4% TO2.1% BERYLLIUM, 0.17% TO 0.42% OF METAL SELECTED FROM THE GROUPCONSISTING ESSENTIALLY OF COBALT, NICKEL, AND MIXTURES THEREOF, AND THEBALANCE OF COPPER, TO EFFECT A PREDETERMINED REDUCTION AND THEREBY TOINCREASE THE TENSILE STRENGTH, WHEREBY A CONCURRENT REDUCTION IN PRECENTELONGATION OCCURS, THEN HEATING THE ROLLED SHEET IN A SALT BATH TO ATEMPERATURE RANGING FROM ABOUT 525*F. TO 825*F., AND MAINTAINING THESHEET HEATED IN SAID TEMPERATURE RANGE IN SAID BATH FOR A PERIOD FROMABOUT TWO SECONDS TO ABOUT FIVE MINUTES, THEN REMOVING THE SHEET FROMTHE BATH, AND IMMEDIATELY UPON REMOVAL, QUENCHING THE SHEET WITH ALIQUID BATH, WHILE MAINTAINING THE SHEET UNDER TENSION CONTINUOUSLYTHROUGHOUT THE HEATING AND QUENCHING OPERATIONS.